This argument is as old as the hills. The best response I have seen was given in a letter by J R R Tolkien written in 1959 in response to a similar query addressed to "any Professor of English Language" at Oxford asking whether "a number of office walls has been damaged" or "have been damaged" was correct: "The answer is that you can say what you like. Pedantry insists that since number is a singular noun, the verb should be singular (has). Common sense feels that since the walls is plural, and are really concerned, the verb should be plural (have). You may take your choice." On a purely personal level I adopt whichever seems more natural.